svchost.exe running 99% cpu

Is anyone else out there having a similar problem ?
The system is fully updated with in both operating system and Norton anti-virus.
Yet after the system sits idle for awhile I notice that it slows right down , I look in the task manager and notice that on instance of 3 of svchost.exe is running at 99% cpu time …this can’t be normal . I have done a little bit of research on the net and some places mention that it could be a virus .
I have tryed to find this , but no anti-virus program can seem to Identify that this is the case .
Any suggestions ?